1. Field of the Invention
The present invention relates to a wireless communication system for multicast communications.
2. Description of the Related Art
The widespread use of wireless LANs has made the communication environments much more convenient than when only wired LANs were used. Particularly, wireless LANs are more suitable to broadcast communications and multicast communications than wired LANs because radio waves from an access point reach all terminals that are positioned in the communication range from the access point. Communications with all terminals in the communication range will be referred to as broadcast communications, and communications with certain terminals in the communication range will be referred to as multicast communications.
Wireless LANs employ a packet communication system wherein the destination of each packet is indicated by a destination address. For making broadcast or multicast communications possible, it is necessary to prepare a broadcast or multicast address and set it for each terminal in addition to the address inherent in each terminal.
Regardless of whether terminals belong to a wired LAN or a wireless LAN, each of the terminals initially has two MAC addresses, i.e., an inherent address and a broadcast address. The broadcast address is commonly used by all the terminals, and comprises 48 bits each represented by “1”. Each terminal receives only packets having its own inherent address or the broadcast address as the destination address, and discards other packets. In this manner, each terminal avoids the reception of unwanted packets.
For multicast communications, it is necessary for each terminal to select, in addition to the inherent address and the broadcast address that are initially set, a MAC address that is not the same as those of any other terminals, and use the selected MAC address as a multicast address.
According to the standards of IEEE802.11, a multicast address is defined. If a multicast address is used in a system, it is necessary to guarantee by some means that the same multicast address is not used in other systems. Therefore, basically, centralized management is indispensable with regards to the allocation of multicast addresses, and individual systems are not allowed to select and use multicast addresses at their discretion.
There has heretofore been proposed a process of generating multicast addresses that do not overlap inherent addresses and broadcast addresses of terminals (see, for example, JP-A-2003-134118).
According to the processed disclosed in JP-A-2003-134118, a hypothetical MAC address for use as a multicast address is generated using a vendor code that is not supposed to be used under normal circumstances. The process makes it possible to generate multicast addresses that do not overlap inherent addresses and broadcast addresses of terminals.
However, the conventional process suffers the following problems:
Some terminals that are available at present allow a MAC address to be set manually. According to the processed disclosed in JP-A-2003-134118, there is a possibility that a generated hypothetical MAC address, though it contains a vendor code that is not supposed to be used, may overlap a MAC address that is actually used in the network.
The Ethernet may be used to perform multicast communications. However, Ethernet multicast communications are basically to be used in cooperation with IP multicast communications. Though Ethernet multicast communications are defined according to standards, they may possibly be implemented in various forms as they are used less frequently. It is doubtful if the Ethernet multicast scheme can be applied to wireless LANs or not.
According to specifications of wireless LANs, a multicast frame format is defined separately from a broadcast beacon frame format. Such a multicast frame format may be used for multicast communications. However, only the multicast frame format is defined, and specific details are not given about how to establish multicast groups. Consequently, it is uncertain whether currently installed functions are capable of performing multicast communications or not.
Multicast communications may be accomplished based on the fact that there are terminals which are capable of receiving beacon frames and broadcast frames of the ordinary data type and terminals which are incapable of receiving such beacon frames and broadcast frames. If a terminal that is capable of receiving beacon frames and broadcast frames of the ordinary data type is used as a multicast communication destination, then it is possible to perform communications with certain terminals. However, various terminals that are incapable of receiving such beacon frames and broadcast frames may possibly be caused to malfunction by those frames.
As described above, the conventional wireless LANs require some centralized management for setting multicast addresses and may possibly adversely affect other systems and terminals. Therefore, the conventional wireless LANs have not effectively be utilized to perform multicast communications that are suitable for use as wireless communications.